1. Field of the Invention
The present invention relates to an apparatus for recording and/or reproducing information on an optical recording medium by irradiation of a light beam, and, more particularly to an optical information recording and/or reproducing apparatus provided with a means for obtaining an on-track/off-track signal (a track count signal) from reflected light or transmitted light of the light beam.
2. Description of the Prior Art
FIG. 1 is a schematic view which illustrates an example of the structure of a conventional optical information recording/reproducing apparatus. Referring to this drawing, reference numeral 10 represents an optical disk having a surface in which spiral or concentric tracks are formed. This optical disk 10 can be rotated by a spindle motor 11 so that information can be recorded or reproduced as a result of the application of the light beam from an optical head 21. The optical head 21 includes a semiconductor laser 12 which serves as the light source. The light beam emitted from this semiconductor laser 12 is converged on the optical disk 10 after it has passed through a collimator lens 13, a beam splitter 14, and an objective lens 15. The beam reflected from the optical disk 10 is again passed through the objective lens 15, and is separated from the incident beam by the beam splitter 14. Then, the thus-separated beam is detected by a light detector 18 after it has passed through a sensor lens 23. The light receiving surface of this light detector 18 is so divided into a plurality of regions as to have a focusing signal and a tracking signal detected in an autotracking (AT) and autofocusing (AF) control circuit 19 by a known method such as the astigmatism method, the push-pull method, or the like on the basis of the output from each of the above-described divided regions. The method of detecting the control signal of the type described above has been disclosed in, for example, U.S. Pat. No. 4,410,969.
The thus-detected control signal is fed back to a lens actuator 16 so that the autotracking and the autofocusing are enabled by moving the objective lens 15 in the direction of the optical axis of the light beam and in the direction which is perpendicular to the optical axis and also which intersects the track.
In order to access the light beam to a desired track in the apparatus of the type described above, a structure can be employed which is arranged such that the overall body of an optical head 21 is moved by a linear motor 22 in the radial direction of the disk, that is, in the direction which intersects the track. Alternatively, another structure can be employed which is arranged such that the objective lens 15 is moved in the direction which intersects the track by inputting a track jump signal to the lens actuator 16. In order to detect the position or the moving direction of the light beam, an on-track/off-track signal is used in the apparatus of this type. The on-track/off-track signal can be obtained by processing, by an on-track/off-track detection circuit 20, the total signal transmitted from all of the regions in the light detector 18. The utilization of such an on-track/off-track signal has been disclosed, for example, in U.S. Pat. No. 4,106,058.
FIG. 2 is a schematic view which illustrates an example of the conventional on-track/off-track detection circuit. An output (a total signal) 1 from the light detector 18 having the waveform as designated by a symbol S shown in FIG. 3 is subjected to a comparison made, by a comparator 6, with a predetermined reference value C which is approximated to the average of the output 1 and transmitted from a constant-voltage source 8. As a result, an on-track/off-track signal 7 as shown in FIG. 3B is transmitted.
However, in the above-described conventional devices, the total signal from the light detector 18 involves, as designated by the symbol S shown in FIG. 3A, a certain offset with respect to a ground level G and becomes a signal modulated by the track when the light beam intersects the track with the focus servo actuated. Furthermore, the offset and the degree of modulation can be changed corresponding to the change in the reflectance of the disk or the change in the laser power. Therefore, the conventional devices encounter a problem in that the on-track/off-track signal which serves as the output cannot be obtained with satisfactory accuracy, be maintained or, in the worst case, the determination of the on-track and off-track cannot be performed if the change in the offset and in the modulation degree is too large since the comparison voltage is arranged to be a constant level.